1. Field of the Invention
This device pertains generally to a device for generating X rays and more specifically to an X-ray transmission cathode wherein the X rays produced in an evacuated X-ray tube by an anode or sample are allowed to exit the tube through the cathode.
2. Description of the Related Art
The typical configuration for a sealed X-ray tube involves a resistively heated, drawn wire filament cathode for generating free electrons in vacuum, and a metallic anode held at high voltage with respect to the cathode. The emitted electrons are electrostatically accelerated to high energy and made to collide with the anode, which then emits the X rays. The voltages required for economical X-ray emission exceed the binding energy of inner electrons in the atoms of the anode, typically kilovolts. The anode emits continuum bremstrahlung X rays as well as characteristic X rays. Emission occurs in all directions, but the intensity in any direction is modified by the absorption of the X rays as they depart their points of origin. The characteristic rays are distinctive for each of the chemical elements, and form the basis of the well known elemental analysis by X-ray emission. Selective detection, processing, and display techniques have been used to record the characteristic rays and analyze the spatial variations of composition in X-ray emitting materials.
As used herein, an x ray photon is a photon with sufficient energy to ionize a neutral atom by photoelectric absorption. There is a wide variation in the energy range of ionizing photons.
The usual geometry for sealed X-ray tubes 10, as shown in FIG. 1a, includes a filament cathode 12, an anode 14, and a separate X-ray "window" 16 made of thin material, usually metal, through which the X rays 18 exit the vacuum sealed X-ray tube 10. It is well understood in the art that a fraction of incident x rays 18 are absorbed by any X-ray transmitting window 16 material such as a window 16, and that the suitability of a material as a window 16 is enhanced for smaller magnitudes of that fraction. The filament cathode 12 is connected between a pair of terminals 13 and 17, to a cathode low voltage power supply 22 which supplies current to the cathode 12 to heat the filament cathode 12 and excite electron flow 15. A high voltage power supply 24 is connected to the anode 14 to accelerate the flow of the emitted electrons 15. In this design, the anode 14 placement and shape is subject to two major geometric constraints, (1) maintaining sufficient distance between the anode 14 and other items that the electric fields within the vacuum sealed X-ray tube 21 remain low enough to preclude breakdown and surface currents, and (2) insuring that the window 16 placement is such that X rays 18 are afforded sufficient solid angle to reach the outside of the vacuum sealed X-ray tube 21 with acceptable levels of absorption.
Typical vacuum sealed X-ray tube 10 design of the prior art places the sample or X-ray target 23 and window 16 such that X rays 18 are emitted at or near 90 degrees from the path of the incident electrons. Because X rays are less strongly absorbed than the electrons, angles are commonly chosen such that the electron penetration distance in the anode 14 is shorter than the exit path for emitted X rays 18. X-ray 18 takeoff angles of 6 to 30 degrees (from the surface of the anode 14) are not uncommon; appreciable X-ray absorption in the anode 14 occurs at these low angles.
A variant among tube designs of the prior art is the transmission anode, end window, tube 20, as shown in FIG. 1b, commonly known as the end-window tube , in which the transmission anode 26 functionalities of an anode and a window are combined in a single member. A transmission anode 26 must allow the electrons 29 to strike the anode 26 to produce X rays 31, dissipate charge and heat from its surfaces and from throughout its volume, and permit the X rays 31 to pass through to the outside; these requirements are usually achieved with transmission anodes 26 made of thin metal foils. The transmission anode, end-window tube 20 is advantageous in some applications, but the requirement for a thin anode 26 results in lower X-ray 31 output power. It is quite common for the end-window anode 26, an exterior component, to be held at ground potential, which leads to the requirement for the cathode 33 portion to be at high voltage. The cathode filament current power supply 34 must float at high negative voltage while the anode 26 is connected to a tube high voltage power supply 32 to accelerate the flow of emitted electrons 31.
In contrast to tube designs of the prior art shown in FIGS. 1a and 1b, the transmission cathode, end-window, tube discussed below, enables the X rays 31 from the transmission cathode 33 to exit the anode 26 at the same angle that the electrons 29 are incident, thus reducing the X-ray absorption and enhancing tube 2 output and permitting grounded exterior components. The transmission cathode 33 is not bombarded by high energy electrons and need not dissipate as much charge or heat from within its volume, thus it need not be as good a volume conductor of either.
While the hot filament cathode based on thermionic emission is very common, alternative technologies based on field emission, photo emission, and plasma emission have been investigated as well. Field emission tips have been used for X-ray production in the past on radiography machines to produce nanosecond pulses of X rays by accelerating electrons from an array of emitters into a metal foil end-window anode. Photoemission involves irradiating the cathode with suitable light sources capable of stimulating the cathode to emit electrons. SEE, U.S. Pat. No. 5,042,058, Rentzepis, issued Aug. 20, 1991, entitled ULTRASHORT TIME-RESOLVED X-ray SOURCE. Plasma emission cathodes involve locally heating the cathode surface to temperatures sufficient to produce a plasma, from which electrons are emitted. SEE, U.S. Pat. No. 5,335,258, Whitlock, issued Aug. 2, 1994, entitled SUBMICROSECOND, SYNCHRONIZABLE X-ray SOURCE.
Spatial resolution based on direct X-ray emission has been practiced with the electron microprobe and scanning electron microscope. Fluorescent X-ray emission has also been used for compositional mapping. SEE, U.S Pat. No. 5,742,658, Tiffin et al., issued Apr. 21, 1998, entitled APPARATUS AND METHOD FOR DETERMINING THE ELEMENTAL COMPOSITIONS AND RELATIVE LOCATIONS OF PARTICLES ON THE SURFACE OF A SEMICONDUCTOR WAFER.
The focusing and collimation of arrays of micro electron sources has been well documented. SEE, U.S. Pat. No. 4,874,981, Spindt, entitled AUTOMATICALLY FOCUSING FIELD EMISSION ELECTRODE, and Cha-mei Tang ET AL.; PLANAR LENSES FOR FIELD-EMITTER ARRAYS; J. Vac. Sci. Technol. B 13(2), March/April 1995, pp. 571-575.
Due to the unavailability of lenses for X rays, geometric imaging means are commonly used to generate X-ray images. X radiography 30, as shown in FIG. 1c, in which a sample 42 is imaged with X-rays 38, typically uses point projection imaging. A small ("point") source of X-rays 36 emits X-rays 38 spherically outward through the exit window of the tube (not shown). The sample 42 to be radiographed is placed between the X-ray source 36 and the imaging detector 44, e.g., an X-ray film plate used for medical imaging. The spatial resolution of the image is limited by the size of the X-ray point source 36. The achievable X-ray output power cannot exceed the ability of the X-ray tube 37 to absorb the heat load of its internal electron beam within the small focal point from which the X-rays 38 emanate. Where the sample is in close proximity to or contacting the imaging detector (typically X-ray film), the arrangement is called contact radiography and unit magnification is achieved. In typical applications where a magnified image is required, this can be obtained by moving the image plane further from the source and the image becomes a projection radiograph 45. This, in turn, increases the X-ray flux required to achieve an exposure, and places greater demands on the X-ray tube (not shown) and power supply.
Areal X-ray sources are not widely used for imaging, as the common filament cathode X-ray tubes are most conducive to providing small X-ray sources.
X-ray windows must transmit X rays, maintain vacuum integrity as essential to the electron trajectories, and, if needed, allow for the dissipation of charge or heat. The cathodes taught in the prior art do not satisfy these requirements and are insufficiently transmissive to X rays to permit their use as an X-ray window.